most important problems that occurred in these systems. Theobject-oriented database systems, since they are fairly new, do not have a very soundtheoretical solution for many of the issues that arise from their implementations. Here aresome of the problems introduced by the new approach: The object-oriented model contains some concepts whose semantics are still underdiscussion. There is no standard data model and consequently there is no standardmethodology for designing an object-oriented scheme. For the relation systems on thecontrary, the ER diagram is totally acceptable. The query language of the relational systems was base on the mathematical theory of therelational algebra and the relational calculus. There is not something similar for the OODBMS.A lot of effort has been done for the definition of an object-oriented algebra since it is clearthat the relational algebra is inadequate for the support of the object-oriented model. The traditional indexing and locking techniques used should be extended in order to be usedfor object-oriented databases. The composite objects cause a lot of trouble and is still anopen research issue. The complexity of the hierarchies of classes created can be so big that the schemas can behandled with difficulty.The object-oriented systems are very much successful in areas where their predecessorsfailed: The design of the schema can be done in a very direct way since the object-oriented modelis very close to the real world model. On the contrary, the relational design which is based oncanonical forms of the relational system is much more awkward. The maintenance of the database is much easier due to the schema evolution facilities andthe modular design allowed by the object-oriented model. The identity concept that gives one internal pointer to each object throughout its lifeprotects the consistency of the database and helps modeling similar real world entities. In the relational systems, this identi...
